There are two ways you could say this: "There is an increase in the population of China." and "There is an increased population in China." (I think you were going for this one)

Neither us correct. Please try again.

There is an increasing population in China. Or:　China's population/The population of China is increasing.

there's an increased population in China or you can say there's an increasing population in China. in both sentences, increase is considered adjective. To take it as an adverb, you can say there's a number of population in China changing increasingly...I think

I think the native speaker says "China has a growing population." "Increase" in various forms does not sound native except in "China's population has increased from 13 to 14 billion."